The problem in an open-hearth melting furnace is not to reach the desired temperature, but to control the temperature and prevent the roof and walls from melting down.
THE ACID OPEN-HEAKTH PROCESS.
The term acid open-hearth furnace means a regenerative gas furnace used for melting steel, and lined with silieious material (sand). It has been shown that the Bessemer process can be.conducted in a vessel lined with silieious material, or in a vessel lined with basic material, and it has been shown that this difference in lining makes a radical difference in the process. In the same way the manner in which a steel melting furnace is lined profoundly influences the subsequent operations. Contrary to popular belief, the bottom in itself plays very little part and has very little influence, but the character of the bottom determines the character of the slag that can be carried, and the character of the slag determines the chemistry of the process.
In the acid open-hearth process a mixture of pig-iron and scrap is charged into the furnace and melted. Nothing is added to form a slag, as the combustion of the silicon and manganese, together with some iron that is oxidized, and some sand from, the bottom, affords a sufficient supply. The slag is about half silica (Si02), while the other half is composed of oxides of iron and manganese. When the mass is melted it is fed with, iron ore, and the oxygen in the ore oxidizes the excess of carbon until the required composition is attained, whereupon the steel is tapped, the proper additions of manganese being made at the time of tapping. Melted spiegel iron, so generally used in Bessemer practice, is not used in open-hearth work, but the manganese is added, in the form of a rich ferromanganese, which is generally thrown into the ladle as the heat is tapped. Sometimes a spiegel iron is used, but this is put into the furnace a little while before tapping and allowed to melt.
It is necessary for the highest success of the operation that the slag should be kept within certain limits in regard to its chemical composition, for if it contains too much silica it is thick and gummy, and the operation will be much retarded, while if it contains too much oxide of iron it will be sloppy and the metal will be frothy and over-oxidized. It would seem at first sight that there would be considerable difficulty in regulating the composition of a slag that is constantly receiving iron ore and constantly absorbing